In general, a secondary battery system such as a power supply device, a distributed power storage device, and an electric automobile using a secondary battery as power storage means is provided with a battery control circuit that manages a battery state. Representative examples of an index which is used for the battery control circuit to manage the battery state include a state of charge (SOC), a state of health (SOH), a permissible current, and a permissible power. The SOC indicates to what extent the battery is charged (what amount of charges dischargeable remains in the battery), and the SOH indicates to what extent the battery deteriorates from an initial state. The permissible current is a maximum value of a chargeable/dischargeable current of the battery, and the permissible power is a value which is obtained by multiplying the permissible current by a battery voltage.
The permissible current of the battery includes a permissible charging current indicating the permissible current in charging and a permissible discharging current indicating the permissible current in discharging. When the SOC is high, the permissible charging current decreases and the permissible discharging current increases. On the other hand, when the SOC is low, the permissible charging current increases and the permissible discharging current decreases. When the battery control circuit can correctly detect the permissible currents, various types of secondary battery systems can maximally utilize charging/discharging performance of a battery. Accordingly, it is extremely important to establish a high-accuracy permissible current detection method.
As a technique of detecting a permissible current, a battery chargeable/dischargeable power estimating method described in PTL 1 has been proposed. In this technique, a current value of a charging/discharging current flowing in a battery is detected, an open voltage of the battery is calculated on the basis of a current-integrated capacity calculated from the detected current value of the charging/discharging current, and chargeable/dischargeable power in a predetermined time from a charging/discharging changing point of the battery is estimated on the basis of the detected current value and the calculated open voltage.